Does quantum mechanics obey causality?

  • I
  • Thread starter n01
  • Start date
  • #76
2,800
602
I'm always puzzled by the question, what the "measurement problem" is? The experimentalists around me have practical problems to solve when they want to measure various things accurately, but there's no real unsolved fundamental problem with measurements out there. They construct marvelous devices to measure things, and that's how the physical quantities are in fact defined, not by quantum theory (or any other theory for that matter). Theory has to describe (predict) what is (will be) measured, if it is a good theory (at least in some limited range of applicability). That's what quantum theory (in the minimal statistical interpretation) provides with an astonishing success. So I don't see, where there is a problem with it, particularly I don't see any "measurement problem".
The problem is, how do we get a definite outcome in macroscopic experiments while the world is fundamentally quantum mechanical?
Interpretations are there to answer this question. MW and dBB seem to solve this. For ensemble interpretation the solution seems to be what bhobba suggests: improper mixtures are the same as proper mixtures. But the difference between ensemble interpretation and MW and dBB interpretations is that ensemble interpretation only works for ensembles. So my problem is, how do we get a definite outcome for a macroscopic experiment on an individual system?
 
  • #77
9,987
3,092
But the difference between ensemble interpretation and MW and dBB interpretations is that ensemble interpretation only works for ensembles. So my problem is, how do we get a definite outcome for a macroscopic experiment on an individual system?

Its a frequentest view. Ensembles is just one way of doing it.

But here really is not the place to discuss it.

Start another thread.

Thanks
Bill
 
  • #78
vanhees71
Science Advisor
Insights Author
Gold Member
19,096
9,897
Well, why do you get a definite outcome when throughing dice although these outcomes are random? Where is the problem with this everyday phenomenon? It's just a very common observable fact that outcomes of measurements are definite after they have occured although they are not predictable and thus are described in terms of probability theory and statistics.

How do you think MW solves your apparent problem? It just adds "parallel universes" to the picture whose existence cannot be observed. It's an element of the interpretation which might be amusing to some philosophers. For physics it's irrelevant. The same holds for dBB: It adds unaobservable trajectories to the picture of interpretation.

Our measurement devices are constructed such as to give definite outcomes in individual experiments although these outcomes are not predictable since the corresponding observables of the quantum system have indefinite values if the system is not prepared in a way that they have definite values. QT also tells you that it is impossible to prepare a system in a state where all its observables take definite values. Through the interaction of a measurement device you get however always a definite pointer outcome, and this pointer outcome is interpreted as a definite value for the measured observable. Otherwise the apparatus is not taken as a good device to measure this observable. That's all.
 
  • #79
2,800
602
When I think in terms of Bayesianism, then everything falls into place and now I can accept the ensemble interpretation as an interpretation on equal footing as MW and dBB and so I'm OK with it now.
Well, why do you get a definite outcome when throughing dice although these outcomes are random? Where is the problem with this everyday phenomenon? It's just a very common observable fact that outcomes of measurements are definite after they have
occured although they are not predictable and thus are described in terms of probability theory and statistics.
This isn't a good analogy and actually betrays your point. This phenomenon is not strange to us because we know the randomness is only emergent and fundamentally the dice follows classical mechanics and so it has a definite outcome whether we can predict it or not.
But if you suggest this is a good analogy to justify the ensemble interpretation, it means you're suggesting exactly what I said, that you need a hidden variable theory to justify the ensemble interpretation.(A claim that I'm now taking back!)
As I said, I'm now OK with the ensemble interpretation if we use Bayesian probability theory and have no problem with it but this is a bad analogy for justifying it!

How do you think MW solves your apparent problem? It just adds "parallel universes" to the picture whose existence cannot be observed. It's an element of the interpretation which might be amusing to some philosophers. For physics it's irrelevant. The same holds for dBB: It adds unaobservable trajectories to the picture of interpretation.
I didn't say I like them, I just said they provide solutions!

Our measurement devices are constructed such as to give definite outcomes in individual experiments although these outcomes are not predictable since the corresponding observables of the quantum system have indefinite values if the system is not prepared in a way that they have definite values. QT also tells you that it is impossible to prepare a system in a state where all its observables take definite values. Through the interaction of a measurement device you get however always a definite pointer outcome, and this pointer outcome is interpreted as a definite value for the measured observable. Otherwise the apparatus is not taken as a good device to measure this observable. That's all.
The fact that part of a physical phenomenon is man-made doesn't mean physics doesn't have to explain it! I don't see how this can be an argument in favor of ensemble interpretation!
But don't bother coming up with further arguments because as I said, I'm now OK with it.
 
  • #80
vanhees71
Science Advisor
Insights Author
Gold Member
19,096
9,897
Physics doesn't explain but describes observed facts (and sometimes makes predictions of observable facts).

The only difference between classical deterministic theory and quantum theory is that in the former randomness is only due to a lack of information of the state of the system and in the latter it's that even the full knowledge of the state does not imply that all observables are determined. This is hard for many to believe, but that's how nature seems to be. Why should it be deterministic? There's no plausible reason for that to be a true property of nature, and as it seems, quantum theory is a more comprehensive description of nature than classical deterministic theories. There is no need for hidden variables that in some way restores determinism, because it's simply not observed in nature! To the contrary all the many experiments demonstrating the violation of Bell's inequality (taken together with the very persuasive assumption os locality of interactions) proof this view wrong. The apparent classical deterministic behavior of macroscopic systems is the emergent phenomenon, not the irreducible randomness according to QT!
 
  • #81
2,800
602
Physics doesn't explain but describes observed facts (and sometimes makes predictions of observable facts).

The only difference between classical deterministic theory and quantum theory is that in the former randomness is only due to a lack of information of the state of the system and in the latter it's that even the full knowledge of the state does not imply that all observables are determined. This is hard for many to believe, but that's how nature seems to be. Why should it be deterministic? There's no plausible reason for that to be a true property of nature, and as it seems, quantum theory is a more comprehensive description of nature than classical deterministic theories. There is no need for hidden variables that in some way restores determinism, because it's simply not observed in nature! To the contrary all the many experiments demonstrating the violation of Bell's inequality (taken together with the very persuasive assumption os locality of interactions) proof this view wrong. The apparent classical deterministic behavior of macroscopic systems is the emergent phenomenon, not the irreducible randomness according to QT!

On this issue, I'm completely with you. I really don't understand people who think common sense is a good judge for understanding phenomena that are not at all common to our sense.
 

Related Threads on Does quantum mechanics obey causality?

Replies
4
Views
2K
  • Last Post
Replies
13
Views
4K
Replies
5
Views
2K
Replies
52
Views
13K
Replies
7
Views
2K
  • Last Post
Replies
3
Views
538
Replies
28
Views
4K
  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
5
Views
2K
Top